Hager was born in Levin to a middle-class "socially aware" family.[1] His father was from Vienna, Austria, a clothing manufacturer who emigrated to New Zealand as a refugee from the Nazis, and his mother was born in Zanzibar (part of Tanzania), and later grew up in Kenya and Uganda.[1]

Hager is an investigative journalist who has written six books. As of February 2015[update], the International Consortium of Investigative Journalists (ICIJ), an international network that has 165 investigative reporters in over 65 countries, has Hager as its only New Zealand member.[4][5]

Hager works on a number of projects at any given time, gathering information about them and looks for sources and mentors. Some projects "come together" but others can continue for a number of years.[6]

Secret Power – New Zealand's Role in the International Spy Network, published in 1996, was Hager's first book.

The book investigates various spying techniques, including Signals Intelligence (Sigint), a form of electronic eavesdropping between countries. The information was taken from interviews with staff in New Zealand's Sigint agency, the Government Communications Security Bureau (GCSB), who revealed the workings of the agency in minute detail: the intelligence targets, equipment, operating procedures, security systems and training, as well as the staff and layout of the intelligence agency's facilities. The book makes special mention of GCSB's facilities at Waihopai and Tangimoana.

It revealed aspects of New Zealand's participation in the UKUSA Agreement facilitating intelligence gathering and sharing between the United States, United Kingdom, Canada, Australia and New Zealand. Since the Edward Snowden revelations, this partnership has become more commonly known as the Five Eyes.

As a result, the insights into the inner workings of the GCSB provide information about the allied agencies as well. In particular, Hager documented the US-coordinated ECHELON system, through which the five agencies intercept and process huge volumes of international e-mail, fax and telephone communications. Hager was one of the earliest to write about the secretive ECHELON worldwide electronic spy network. As a result of his book, in 2001 he testified before the European Parliament on his research into the network.[7]

The material is based on a large amount of documentation leaked by a staff member from the local branch of Shandwick (now Weber Shandwick Worldwide),[8] a global public relations company, which had been hired by Timberlands to run a secret campaign against environmental groups such as Native Forest Action between 1997 and 1999.

The book describes its tactics of surveillance of meetings, monitoring the press and responding to every letter to the editor, greenwashing, the use of strategic lawsuit against public participation (SLAPPs), cleaning anti-logging graffiti and blotting out campaign posters in public places, and managing to install its pro-logging educational materials into schools.

The year after the book’s publication, the new Labour government passed the Forests (West Coast Accord) Act 2000, which halted the logging of publicly owned native forests and provided a $120 million transition package for the West Coast economy.

Seeds of Distrust: The Story of a GE Cover-up was a study of government processes and decision-making under New Zealand's Labour-led government.

The book details an incident in November 2000, during the Royal Commission on Genetic Modification, when government officials were alerted to evidence of an accidental release of genetically modified corn plants, which was illegal under New Zealand law. The book traces the stages of industry lobbying and government decision making leading up to a decision to regard the incident as insignificant and keep it secret from the public.

The book was released in July 2002, just as a snap election was called, and helped make genetically modified organisms a major election issue.

At a media conference of government officials convened immediately after the book's release, the Chief Executive of the New Zealand Ministry for the Environment, Barry Carbon, conceded that the book was largely accurate but disputed the interpretation of the material.[9]

The Christchurch newspaper The Press studied the official documents and concluded:

“

"So who's telling the truth, Hager or the Government? Officials at a special briefing for journalists last week memorably commented that they did not disagree with most of Hager's facts, just his conclusions. That is unsurprising, given that the conclusions that can be drawn are not palatable ones. The documents raise some serious questions about the level of open government New Zealand really has and the strength of our much-vaunted biosecurity regime."[10]

In November 2006 Hager's book The Hollow Men: A Study in the Politics of Deception was published. The book details a wide range of National Party internal party documents including emails which were leaked by six National Party insiders. The documents were written by the party leader Don Brash and a wide range of people associated with him.

The origin of the book was the exposing of Exclusive Brethren secret funding of National Party election advertising during the 2005 general election. It describes the National Party strategy discussions and the techniques the party advisors used to try to win the 2005 national elections. The thinking behind major speeches, media management, election advertising and election messages is shown in the participants' own words. Many of the party's previously anonymous major donors are identified and relations with them are documented.[citation needed]

The book was initially prevented from being published when Brash obtained an injunction preventing anyone from publishing material from emails that had been stolen from him. At the time Brash was unaware that Hager had completed and was about to launch his book.[11] However, on 23 November 2006, Brash announced his resignation from the leadership of the National Party.[12] The following day he had the injunction lifted, allowing the book to be released.[citation needed]

The Hollow Men was made into a theatrical piece by playwright Dean Parker.[13] Hager assisted with the feature-length documentary of the book, made by Alister Barry, that was released in July 2008.[14]

In August 2011 Hager published Other People's Wars, an investigation into New Zealand's role in the "War on Terror". Using leaked military information, Hager highlights the difference between what New Zealand was told about its role in Afghanistan and Iraq, and what the leaked documents show.

The book demonstrates the role of public relations in blurring the lines between the idea of military as a public service and the agendas that the New Zealand Defence Force was orchestrating for foreign interests. Hager concludes that New Zealand needs greater accountability and neutrality in its armed forces.[citation needed]

It was recognised internationally, with investigative journalist Seymour Hersh stating in a review, "Nicky Hager has more knowledge and understanding of the American intelligence world in Afghanistan – both its good and its very bad points – than any reporter I know."[15]

Other People’s Wars was also adapted into a play by Dean Parker in 2012.[16]

In August 2014, Hager published Dirty Politics: How attack politics is poisoning New Zealand’s political environment, featuring leaked emails between National Party figures and right-wing bloggers. According to Hager the book aims to tell the story of "how attack politics is poisoning NZ's political environment".[17]

Among the claims in the book are that during the 2011 election campaign a right-wing blogger, Cameron Slater, who is the son of former National Party President John Slater,[18] obtained a database of Labour Party members, emails and donations which were used to attack the Labour Party. Also in 2011 the PM's office used its knowledge of secret SIS documents to tip off a right-wing blogger and arrange an attack on Labour leader Phil Goff, and drafted Official Information Act requests for a right-wing blogger to use in other attacks.[19]

On 21 September 2016, Nicky Hager was subpoenaed by Colin Craig's lawyers to testify as part of the latter's defence in a defamation suit filed by Jordan Williams, the founder of the New Zealand Taxpayers' Union. Craig had read Hager's book Dirty Politics before producing his pamphlet Dirty Politics and Hidden Agendas, which allegedly defamed Williams. Hager took the stand as an expert witness and alleged that Jordan William and Cameron Slater's blog attacks on Craig followed a similar pattern that had been documented in his book. After testifying for fifteen minutes, Hager was allowed to leave by the judge and was not cross-examined by William's lawyers.[20][21]